Optical fibers are fibers that transmit light. Optical fibers are typically made by heating and softening an optical fiber preform in a furnace and then drawing the optical fiber preform to the desired fiber thickness. Optical fiber preforms typically comprise a core rod surrounded by an overclad body. Several conventional systems and methods currently exist for producing such optical fiber preforms and the individual components of the preforms.
Conventional methods for forming the core rod include vapor-phase axial deposition (VAD), outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), and plasma chemical vapor deposition (PCVD). The overclad body may then be formed directly on the exterior surface of the core rod, such as by OVD, or may be formed independently.
A conventional method for forming the overclad body is as follows: starting materials, typically silicon compounds, such as silicon tetrachloride and/or silicon dioxide particles are deposited on a mandrel or substrate by OVD to form a soot deposition body. The deposition continues until the desired body size is attained. After the deposition is completed, the mandrel or substrate is removed. The soot deposition body is then dehydrated and subsequently vitrified in a heating furnace to form a quartz glass cylinder with a bore extending therethrough. Finally, the quartz glass cylinder may be subjected to some manner of machining, such as polishing of the interior and exterior surfaces the cylinder. Such quartz glass cylinders typically have outer diameters of approximately 200 mm and lengths of approximately 3 meters.
The quartz glass cylinder may then be used as an overclad body which is integrated with the core rod to form the optical fiber preform. Conventional methods of forming the optical fiber preform include the Rod-In-Tube (RIT) method and the Rod-In-Cylinder (RIC) method. In the RIT method, the quartz glass cylinder, formed as described above, is drawn down into a plurality of overclad tubes, typically approximately 60 to 90 mm in diameter and approximately 1 to 2 meters in length, which are subsequently integrated with the core rod. In the RIC method, the quartz glass cylinder is used as an overclad cylinder to be integrated with the core rod. Both methods involve inserting the core rod in the quartz glass cylinder or tube, and thus are the same from a technical standpoint. The primary difference between the methods lies merely in the size of overclad tube relative to that of the overclad cylinder.
In the RIT and offline RIC methods, the core rod is inserted into the overclad tube or cylinder and the assembly is heated to a sufficiently high temperature so as to cause the overclad tube or cylinder to collapse on and adhere to the core rod, thereby yielding an optical fiber preform. In the offline RIC method, the assembly is also stretched during collapse of the overclad cylinder onto the core rod in order to form the optical fiber preform. The resulting preforms are then sent to a draw tower for formation of an optical fiber. In the online RIC method, the core rod is inserted into the overclad cylinder and the assembly is heated to a sufficiently high temperature so as to cause the overclad cylinder to collapse on and adhere to the core rod to yield an optical preform, and the resulting preform is immediately drawn to yield an optical fiber in a draw tower.
Typically, the optical fiber preforms produced by these methods have outer diameters of approximately 50 to 210 mm and average lengths of approximately 1,000 to 3,000 mm. Typically, the optical fibers produced by these methods have outer diameters of approximately 90 to 125 μm and average lengths of approximately 1,000 to 10,000 km.
However, with conventional RIT and RIC methods, it is difficult to produce preforms or fibers which are free from irregularities, such as voids, airlines and bubbles, and contaminants, such as various foreign matters. Typically, such irregularities and contaminants exist at the interface between the core rod and the overclad tube/cylinder. Such irregularities and contaminants ultimately negatively impact various properties of the resulting optical fiber, such as increased attenuation and light scattering losses. Accordingly, it would be beneficial to provide improved RIT and RIC systems and methods for producing optical fiber preforms and optical fibers free from contaminants, impurities and atomic defects.